SVI arbitrage constraints.

An options surface can look smooth while still implying impossible prices. The fit layer checks the SVI smile and the surface assembled across expiries before the UI treats it as a production-quality curve.

For each expiry, the fitted total variance smile should not imply a negative risk-neutral density. In practical terms, the engine evaluates the smile shape across a grid of log-moneyness points and publishes g-test diagnostics when local convexity or wing behavior becomes suspicious.

At the same moneyness, total variance should generally not fall as expiry increases. The surface grid and tenor rows make that check visible by comparing the fitted BTC smiles across maturities rather than only one expiry at a time.

The Fit tab exposes the fitted variance curve, fitted volatility curve, 3D surface projection, and g-test panel. If the g-test view spikes or the fit error deteriorates, it is a signal to inspect the market data, stale venue overlays, or calibration inputs before relying on the surface.

The variance view plots total implied variance by expiry rather than annualized implied volatility. SVI is calibrated in variance space, so these lines are the native fitted objects.

The g-test view is a diagnostic series published by the upstream engine for the fitted SVI smile. A stable line is expected for a usable fit; spikes or sign changes should be reviewed alongside stale quotes, crossed markets, and current fit error.

The same fit-quality state is available in request-based WebSocket evaluation for the live BTC surface. REST behavior and other configured currencies remain testing scope and are not presented as stable public coverage.